Lithium composite phosphate-based compound and preparation method therefor

ABSTRACT

Provided is a porous lithium composite phosphate-based compound containing lithium and having open pores formed in primary particles. As the open pores are formed in the primary particles themselves, a contact area between an electrolyte and the lithium composite phosphate-based compound is maximized, and low conductivity is compensated for, such that a diffusion rate of lithium ions is remarkably increased, and when the lithium composite phosphate-based compound is used as an active material of a secondary battery, the secondary battery may be charged and discharged at a high speed. Additionally, there are advantages in that an electrode density may be significantly increased in addition to the increase in the diffusion rate of the lithium ions, and charge and discharge cycle characteristics may be significantly stable.

TECHNICAL FIELD

The present invention relates to a lithium composite phosphate-basedcompound, uses thereof, and a preparation method therefor.

BACKGROUND ART

A lithium phosphate-based compound has significantly excellent stabilitydue to a structural feature even in a case in which short-circuit orover-heating of a battery occurs. A lithium phosphate iron-basedcompound having an olivine structure may be prepared using a cheap rawmaterial and have high bulk density and excellent thermal stability andlife characteristics, such that the lithium phosphate iron-basedcompound has been commercialized as a cathode active material.

However, in a lithium composite phosphate salt having an olivinestructure, lithium ions may be diffused only in one direction, such thatresearch for improving a diffusion rate of lithium has been conducted asin International Patent NO. WO11/132930.

DISCLOSURE Technical Problem

An object of the present invention is to provide a lithium compositephosphate-based compound suitable for a cathode active material of asecondary battery and a preparation method therefor. More particularly,the object of the present invention is to provide a lithium compositephosphate-based compound capable of significantly increasing a diffusionrate of lithium ions and having significantly high tap density inaddition to an increase in diffusion rate of the lithium ions and apreparation method therefor.

Technical Solution

In one general aspect, a lithium composite phosphate-based compoundcontains lithium and includes a porous lithium composite phosphate-basedcompound having open pores formed in primary particles.

A Brunauer-Emmett-Teller (BET) specific surface area of the primaryparticle may be 25 to 50 m²/g.

A pore volume of the primary particle may be 0.1 to 0.25 cm³/g.

The primary particle may contain nano pores having an average pore sizeof 1 to 5 nm.

The primary particle may contain at least pores having a bimodal sizedistribution.

The lithium composite phosphate-based compound may have a compositionrepresented by the following Chemical formula 1.

Li_(1+b)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1)

(In Chemical Formula 1, M is one or more selected from the groupconsisting of Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al,and Ag, and x, a, and b are real numbers satisfying 0.00≦x≦1,0.00≦a≦0.1, and 0.00≦b≦0.10, respectively.)

A size of the primary particle may be 10 nm to 200 μm.

The lithium composite phosphate-based compound may be a cathode activematerial for a secondary battery.

In another general aspect, a cathode active material for a secondarybattery contains the lithium composite phosphate-based compound asdescribed above.

A carbon coating layer may be formed on a surface of the primaryparticle of the lithium composite phosphate-based compound, wherein thesurface of the primary particle may include a surface by the open pore.

The cathode active material may further contain 2 to 6 parts by weightof carbon based on 100 parts by weight of the primary particle of thelithium composite phosphate-based compound.

In another general aspect, a cathode for a secondary battery containsthe cathode active material as described above.

In another general aspect, a lithium secondary battery contains thecathode as described above.

In another general aspect, a preparation method for a lithium compositephosphate-based compound includes: a) injecting and stirring at least achelating agent, a precursor of phosphate, a precursor of a first metalincluding lithium, and a water-insoluble precursor of a second metal ina polar solvent to prepare a precursor dispersion solution; and b)heat-treating precursor powder obtained by concentrating and drying theprecursor dispersion solution.

The second metal may be one or two or more metals selected amongtransition metals, and in the water-insoluble precursor of the secondmetal, the second metal may have a valence of 2 (+2).

The precursor powder may contain a complex of the second metal having avalence of 3.

The precursor of the first metal may be a water-soluble precursor or awater-insoluble precursor.

The second metal may include iron.

A water-soluble precursor of a third metal, one or more metals selectedamong Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al, and Agmay be further injected into the polar solvent.

The lithium composite phosphate-based compound may include a phosphatesatisfying a composition represented by the following Chemical Formula1.

Li_(1+a)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1)

(In Chemical Formula 1, M is one or more selected from the groupconsisting of Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al,and Ag, and x, a, and b are real numbers satisfying 0.00≦x≦1,0.00≦a≦0.1, and 0.00≦b≦0.10, respectively.)

The concentration and drying may be performed by vacuum concentration.

The vacuum concentration may be performed at a temperature of 30 to 60°C. and a pressure of 20 to 100 mbar.

The heat-treatment may be performed under a reduction atmosphere or aninert atmosphere.

The heat-treatment may be performed at 400 to 800° C.

Advantageous Effects

In a lithium composite phosphate-based compound according to the presentinvention, as open pores are formed in primary particles themselves, acontact area between an electrolyte and the lithium compositephosphate-based compound may be maximized, and lower conductivity may becompensated for, such that a diffusion rate of lithium ions may beremarkably increased.

As a secondary battery containing a lithium composite phosphate-basedcompound according to the present invention contains a lithium compositephosphate-based compound having open pores formed in primary particlesthemselves, the secondary battery may be charged and discharged at ahigh speed. In addition, as the primary particles of the lithiumcomposite phosphate-based compound have a size of several ten nanometersto several hundred micrometers and the open pores are formed, thebattery may have advantages in that an electrode density may beremarkably increased in addition to an increase in the diffusion rate ofthe lithium ions, and charge and discharge cycle characteristics may besignificantly stable.

A preparation method for a lithium composite phosphate-based compoundaccording to the present invention has advantages in that a lithiumcomposite phosphate-based compound having a porous structure in whichopen pores are formed in primary particles may be prepared by a simpleprocess of mixing and stirring a chelating agent and a precursor of eachof the metals of a desired composition with each other, and thenconcentrating, drying, and heat-treating the resultant, and a porouslithium composite phosphate-based compound having a uniform quality maybe mass-produced under mild process conditions.

DESCRIPTION OF DRAWINGS

FIG. 1 is a view illustrating an X-ray diffraction (XRD) pattern of alithium composite phosphate-based compound prepared according to Exampleof the present invention.

FIGS. 2A to 2D are views illustrating results obtained by observinglithium composite phosphate-based compounds prepared according toExamples of the present invention using a high-magnification scanningelectron microscope (SEM).

FIGS. 3A to 3D are views illustrating results obtained by observing thelithium composite phosphate-based compounds prepared according to theExamples of the present invention using a low-magnification scanningelectron microscope (SEM).

FIG. 4 is a view illustrating results obtained by measuring poredistribution of the lithium composite phosphate-based compounds preparedaccording to the Example of the present invention.

FIGS. 5A and 5B are views illustrating Fourier transform-infrared(FT-IR) spectra of precursor powder and the lithium compositephosphate-based compound prepared according to the Example of thepresent invention.

FIG. 6 is a view showing charge and discharge characteristics of a cellmanufactured according to the Example of the present invention.

FIG. 7 is a view showing charge and discharge cycle characteristics ofthe cell manufactured according to the Example of the present invention.

BEST MODE

Hereinafter, a lithium composite phosphate-based compound according tothe present invention, uses thereof, and a preparation method thereforwill be described in detail with reference to the accompanying drawings.Here, technical terms and scientific terms used in the presentspecification have the general meaning understood by those skilled inthe art to which the present invention pertains unless otherwisedefined, and a description for the known function and configurationobscuring the present invention will be omitted in the followingdescription.

The lithium composite phosphate-based compound according to the presentinvention contains a phosphate salt containing lithium, and may containa phosphate salt containing lithium and one or more metal elementsexcept for lithium.

The lithium composite phosphate-based compound according to the presentinvention may be a porous lithium composite phosphate-based compoundcontaining at least lithium and having open pores formed in primaryparticles.

The lithium composite phosphate-based compound according to the presentinvention may have a significantly rapid diffusion rate of Li ions whilehaving a specific surface area significantly increased due to formationof the open pores in the primary particles themselves.

In the lithium composite phosphate-based compound according to anexemplary embodiment of the present invention, the primary particle maymean a crystal particle.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention, the open pore may mean apore having an opening formed on a surface of the primary particle, anda pore of which an interior of a pore formed in the primary particle isfilled with a liquid including an electrolyte through the opening.

The lithium composite phosphate-based compound according to theexemplary embodiment of the present invention may have a compositionrepresented by the following Chemical Formula 1.

Li_(1+a)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1)

In Chemical Formula 1, M is one or more selected from the groupconsisting of Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al,and Ag, and x, a, and b are real numbers satisfying 0.00≦x≦1,0.00≦a≦0.10, and 0.00≦b≦0.10, respectively.

The lithium composite phosphate-based compound according to theexemplary embodiment of the present invention may have a compositionrepresented by the following Chemical Formula 1-1.

Li_(1+a)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1-1)

In Chemical Formula 1-1, M is one or more selected from the groupconsisting of Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al,and Ag, and x, a, and b are real numbers satisfying 0.00≦x<1,0.00≦a≦0.10, and 0.00≦b≦0.10, respectively.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention represented by thefollowing Chemical Formula 1 or 1-1, M may be one or two or more metalsselected among transition metals including Ni, Co, Mn, Ti, Cr, Cu, V,Zn, and Ag; and alkali earth metals including Ca, Sr, Ba, and Mg; M maybe one or two or more metals selected among the transition metalsincluding Ni, Co, Mn, Ti, Cr, Cu, V, Zn, and Ag; or M may be one or twoor more metals selected from the alkali earth metals including Ca, Sr,Ba, and Mg.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention, a Brunauer-Emmett-Teller(BET) specific surface area of the primary particle may be 25 to 50m²/g, and more preferably, 30 to 50 m²/g.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention, a pore volume of theprimary particle may be 0.1 to 0.25 cm³/g, more preferably, 0.14 to 0.25cm³/g. The pore volume of the primary particle may mean a volume of thepore (open pore) per unit mass of the primary particle.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention, the primary particle maycontain a nano pore having an average pore size of 1 to 5 nm.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention, the primary particle maycontain at least pores having a bimodal size distribution. The poreshaving a bimodal size distribution may include a nano pore having anaverage pore size of 1 to 5 nm and a sub-micro pore having an averagepore size of 10 to 30 nm.

In the pores having the bimodal size distribution, a volume ratio of thenano pore and the sub-micro pore may be 100:10 to 80.

In this case, sizes and distribution of the pores present in the primaryparticle may be measured by a general BET method using physicaladsorption.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention, a size (diameter) of theprimary particle may be 10 nm to 200 μm, specifically, 1 μm to 200 μm,and more specifically, 5 μm to 200 μm.

In the lithium composite phosphate-based compound according to theexemplary embodiment of the present invention, the primary particle hasthe size as described above while having open pores formed therein, suchthat a diffusion rate of the lithium ion may be significantly increased,a current density may be increased to 1.0 g/cc or more, andcharge/discharge cycle characteristics may be significantly stable.

As a substantial example, in a case of a lithium secondary batteryincluding a cathode containing primary particles having a size of 1 to200 μm, a change in battery capacity (mAhg⁻¹) at 28th cycle based onbattery capacity (mAhg⁻¹) at 1st cycle (battery capacity at 28thcycle/battery capacity at 1st cycle×100%) (charge and dischargecondition: 4.3-2.5V, 0.1 C) may be maintained to 99.95% or more.

The lithium composite phosphate-based compound according to theexemplary embodiment of the present invention may be a cathode activematerial for a secondary battery. In this case, the secondary batterymay include a lithium ion secondary battery or a lithium polymersecondary battery.

The present invention includes a cathode active material for a secondarybattery, containing the lithium composite phosphate-based compound asdescribed above. In this case, the secondary battery may include alithium ion secondary battery or a lithium polymer secondary battery.

The cathode active material for a secondary battery according to thepresent invention contains the lithium composite phosphate-basedcompound having the open pores formed in the primary particlesthemselves, in a case in which the lithium composite phosphate-basedcompound is a coarse particle, the battery may be charged and dischargedat a high speed, an electrode density in addition to the diffusion rateof the lithium ions may be significantly increased, and charge anddischarge cycle characteristics may be significantly stable.

In the cathode active material according to the exemplary embodiment ofthe present invention, a carbon coating layer may be formed on a surfaceof the primary particle of the lithium composite phosphate-basedcompound as described above. The carbon coating layer may be acontinuous layer or a discontinuous layer. The carbon coating layer maybe formed on at least some region of the surface of the primaryparticle, or the entire region of the surface of the primary particle.Here, the surface of the primary particle may include a surface by theopen pore.

The cathode active material according to the exemplary embodiment of thepresent invention may contain 2 to 6 parts by weight of the carbon as acoating layer based on 100 parts by weight of the primary particle ofthe lithium composite phosphate-based compound as described above.Carbon contained in the cathode active material may form the coatinglayer at least partially covering the surface of the primary particle.Carbon contained in the cathode active material may be a carbon particleor carbon fiber physically attached to the primary particle. Carboncontained in the cathode active material may be a carbon particle and/orcarbon fiber mixed with the primary particle.

The present invention includes a cathode for a secondary battery,containing the cathode active material as described above. In this case,the secondary battery may include a lithium ion secondary battery or alithium polymer secondary battery.

The cathode according to an exemplary embodiment of the presentinvention may include a current collector and an active material layerformed on at least one surface of the current collector and containingthe cathode active material as described above.

The present invention includes a secondary battery provided with thecathode as described above.

The secondary battery according to the exemplary embodiment of thepresent invention may include a lithium ion secondary battery or alithium polymer secondary battery, and further include an anode, anelectrolyte, and a separator in addition to the above-mentioned cathode.

In this case, the anode may contain an anode active material generallyused in the secondary battery, and an example of the anode activematerial may include one or two or more selected among carbon, graphite,silicon, lithium titanium oxide (LTO), and composites thereof.

Here, the electrolyte may include a non-aqueous electrolyte generallyused in the secondary battery, and an example of the electrolyte mayinclude a liquid electrolyte in which a lithium salt including lithiumperchlorate, lithium fluoroborate, or lithium hexafluorophosphate, isdissolved in a solvent, and an example of the solvent may includeester-based solvents including propylene carbonate, ethylene carbonate,dimethyl carbonate, and ethylmethyl carbonate.

Here, the separator may include a separator generally used in thesecondary battery in order to prevent a short-circuit between the anodeand the cathode in the secondary battery, and serve to support theelectrolyte. An example of the separator may include a microporousmembrane containing polyethylene, polypropylene, or polyolefin, and havea stacking structure in which organic membranes such as a plurality ofpolyethylene membranes, polypropylene membranes, non-woven fabrics, andthe like, are stacked in order to prevent an over-current, maintain theelectrolyte, and improve physical strength.

A preparation method for a lithium composite phosphate-based compoundaccording to the present invention may include: a) injecting andstirring at least a chelating agent, a precursor of phosphate, aprecursor of a first metal including lithium, and a water-insolubleprecursor of a second metal in a polar solvent to prepare a precursordispersion solution; and b) heat-treating precursor powder obtained byconcentrating and drying the precursor dispersion solution.

In the preparation method according to an exemplary embodiment of thepresent invention, the first and second metals are metals contained inthe desired lithium composite phosphate-based compound, and the firstand second metals may be metals equal to or different from each other.

As described above, in the preparation method according to the exemplaryembodiment of the present invention, a metal phosphate salt may beprepared by using the water-insoluble precursor of the metal and thechelating agent while preparing a liquid precursor material containingthe precursor of the metal and the precursor of phosphate of the desiredlithium composite phosphate-based compound, concentrating and drying theprecursor dispersion solution to prepare the precursor powder, andheat-treating the precursor powder, thereby making it possible toprepare a metal phosphate salt having a porous structure in which openpores are formed in primary particles. That is, in the preparationmethod according to the exemplary embodiment of the present invention,the porous lithium composite phosphate-based compound in which the openpores are formed in the primary particles may be prepared using thewater-insoluble precursor of the metal, which is not dissolved in thepolar solvent but maintains a solid phase.

In the preparation method according to the exemplary embodiment of thepresent invention, in a case in which the chelating agent and theprecursor of phosphate, and selectively, the precursor of the firstmetal are water-soluble, as the polar solvent, any solvent having apolarity may be used as long as the precursor of the first metaltogether with the chelating agent and the precursor of phosphate isdissolved therein. As a substantial example, the polar solvent mayinclude water, lower-alcohols having 1 to 5 carbon atoms, acetone,formamide, diformamide, acetonitrile, tetrahydrofuran,dimethylsulfoxide, α-terpineol, β-terpineol, dihydroterpineol, ormixtures thereof.

In the preparation method according to the exemplary embodiment of thepresent invention, as the chelating agent, any organic material may beused as long as it is dissolved in the polar solvent and forms a complexwith the first metal and/or the second metal. As a substantial example,the chelating agent may include an organic acid forming a complex withthe first metal and/or the second metal, wherein the organic acid maycontain one or two or more material selected among citric acid, aceticacid, succinic acid, malonic acid, malic acid, oxalic acid, propionicacid, tartaric acid, lactic acid, pyruvic acid, and fumaric acid.

In the preparation method according to the exemplary embodiment of thepresent invention, 5 to 20 parts by weight, preferably, 10 to 15 partsby weight of the chelating agent may be injected based on 100 parts byweight of the polar solvent.

In the preparation method according to the exemplary embodiment of thepresent invention, the precursor of the first metal may be awater-soluble precursor or a water-insoluble precursor.

In the preparation method according to the exemplary embodiment of thepresent invention, the precursor of the first metal may be awater-soluble precursor dissolved in the polar solvent including water.As a substantial example, the precursor of the first metal may include aprecursor having a water-solubility of 1 g or more, more substantially,10 g or more, and further more substantially, 53 g or more, based on 100g of water at room temperature (25° C.) and normal pressure (1 atm).

In the preparation method according to the exemplary embodiment of thepresent invention, the precursor of the first metal may be a salt(including a hydrate) containing the first metal, which is dissolved inthe polar solvent including water, or a complex in which an organicligand is coordinated with the first metal. As a substantial example,the precursor of the first metal may include one or two or morematerials selected among nitrates, sulfates, acetates, citrates,chlorides, sulfites, chloride salts, bromide salts, and iodide salts ofthe first metal, and hydrates thereof.

In the preparation method according to the exemplary embodiment of thepresent invention, the precursor of the first metal may be awater-insoluble precursor that is not dissolved in the polar solventincluding water. As a substantial example, the precursor of the firstmetal may have a water-solubility equal to or less than a detectionstandard of a measurement device, or a water-solubility of substantially0.034 g or less, and more substantially 0.008 g or less, based on 100 gof water at room temperature (25° C.) and normal pressure (1 atm).

In the preparation method according to the exemplary embodiment of thepresent invention, the precursor of the first metal may be a saltcontaining the first metal that is not dissolved in the polar solventincluding water or a complex in which an organic ligand is coordinatedwith the first metal. As a substantial example, the precursor of thefirst metal may include one or two or more materials selected amongphosphates, carbonates, hydroxides, and fluorides of the first metal.

In the preparation method according to the exemplary embodiment of thepresent invention, the water-insoluble precursor of the second metal maybe a precursor of the second metal that is not dissolved in the polarsolvent including water. As a substantial example, the water-insolubleprecursor (including the water-insoluble precursor of the second metal)may include a precursor having a water-solubility equal to or less thana detection standard of a measurement device or less, or awater-solubility of substantially 0.034 g or less, and moresubstantially, 0.008 g or less, based on 100 g of water at roomtemperature (25° C.) and normal pressure (1 atm).

In the preparation method according to the exemplary embodiment of thepresent invention, the water-insoluble precursor of the second metal maybe a salt containing the second metal or a complex in which an organicligand is coordinated with the second metal, which is not dissolved inthe polar solvent including water. As a substantial example, thewater-insoluble precursor of the second metal may include oxalates,acetates, nitrates, or sulfates of the second metal, or mixturesthereof.

In the preparation method according to the exemplary embodiment of thepresent invention, an average particle size of the water-insolubleprecursor of the second metal may be 1 μm to 50 μm, specifically, 1 μmto 30 μm, and more specifically, 3 μm to 18 μm.

In the preparation method according to the exemplary embodiment of thepresent invention, the second metal may be one or two or more metalsselected among transition metals, and in the water-insoluble precursorof the second metal, the second metal may have a valence of 2. That is,the precursor of the second metal may be a precursor of one or two ormore metals selected among the transition metals, and be a precursor ofthe transition metal having an oxidation number of 2.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,the first metal may include lithium, and the second metal may includeiron. The lithium composite phosphate-based compound satisfying ChemicalFormula 1 or 1-1 as described above may be prepared by the preparationmethod according to the exemplary embodiment of the present invention inwhich the first metal includes lithium and the second metal includesiron.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,the precursor of the first metal may be a precursor of lithium, whereinthe precursor of lithium may be a water-insoluble precursor of lithiumor a water-soluble precursor of lithium. As a substantial example, theprecursor of lithium may include lithium phosphate, lithium carbonate,lithium nitrate, lithium sulfate, lithium acetate, lithium citrate,lithium chloride, lithium hydrate, lithium hydroxide, lithium sulfite,lithium fluoride, lithium bromide, lithium iodide, or mixtures thereof.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,the water-insoluble precursor of the second metal may be awater-insoluble precursor of iron, and as a substantial example, thewater-insoluble precursor of iron may include iron oxalate, ironacetate, iron nitrate, an iron metal, iron oxides (including FeO, Fe₂O₃,and Fe₃O₄), or mixtures thereof. An average particle size of thewater-insoluble precursor of iron may be 1 μm to 50 μm, specifically, 1μm to 30 μm, and more specifically, 3 μm to 18 μm.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,iron of the water-insoluble precursor of iron may have an oxidationnumber of 2. That is, the water-insoluble precursor of iron may includea divalent precursor of iron that is not dissolved in the polar solvent.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,as the precursor of phosphate, any water-soluble precursor of phosphatemay be used as long as it is generally used to prepare a phosphate-basedcompound, wherein the water-soluble precursor of phosphate may includeammonium phosphate, phosphoric acid, or mixtures thereof.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,as the precursor of phosphate, a water-soluble precursor of a thirdmetal (M of Chemical Formula 1) corresponding to one or more metalsselected among Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn,Al, and Ag may be further injected into the polar solvent.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,the third metal may be one or two or more metals selected amongtransition metals including Ni, Co, Mn, Ti, Cr, Cu, V, Zn, and Ag; andalkali earth metals including Ca, Sr, Ba, and Mg; the third metal may beone or two or more metals selected among the transition metals includingNi, Co, Mn, Ti, Cr, Cu, V, Zn, and Ag; or the third metal may be one ortwo or more metals selected from the alkali earth metals including Ca,Sr, Ba, and Mg.

In the preparation method according to the exemplary embodiment of thepresent invention, the water-soluble precursor of the third metal may bea precursor of the third metal, which is dissolved in the polar solventincluding water. A substantial example, the water-soluble precursor(including the water-soluble precursor of the third metal) may include aprecursor having a water-solubility of 1 g or more, more substantially,10 g or more, and further more substantially, 53 g or more, based on 100g of water at room temperature (25° C.) and normal pressure (1 atm).

In the preparation method according to the exemplary embodiment of thepresent invention, the water-soluble precursor of the third metal may bea salt (including a hydrate) containing the third metal, which isdissolved in the polar solvent including water, or a complex in which anorganic ligand is coordinated with the third metal. As a substantialexample, the water-soluble precursor of the third metal may include oneor two or more materials selected among phosphates, carbonates,nitrates, sulfates, acetates, citrates, chlorides, hydroxides, sulfites,fluoride salts, chloride salts, bromide salts, and iodide salts of thethird metal, and hydrates thereof.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,the lithium composite phosphate-based compound may satisfy the followingChemical Formula 1, more specifically, the following Chemical Formula1-1. Here, the water-soluble or water-insoluble precursor of lithium(the precursor of the first metal), the water-insoluble precursor ofiron (the precursor of the second metal), the precursor of phosphate,the water-soluble precursor of the third metal (M) may be weighed so asto satisfy the composition represented by the following Chemical Formula1 or 1-1 to thereby be injected into the polar solvent.

Li_(1+a)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1)

In Chemical Formula 1, M is one or more selected from the groupconsisting of Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al,and Ag, and x, a, and b are real numbers satisfying 0.00≦x≦1,0.00≦a≦0.10, and 0.00≦b≦0.10, respectively.

Li_(1+a)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1-1)

In Chemical Formula 1-1, M is one or more selected from the groupconsisting of Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al,and Ag, and x, a, and b are real numbers satisfying 0.00≦x<1,0.00≦a≦0.10, and 0.00≦b≦0.10, respectively.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,when the lithium composite phosphate-based compound is defined byChemical Formula 1 or 1-1, the third metal (M) may be one or two or moremetals selected among transition metals including Ni, Co, Mn, Ti, Cr,Cu, V, Zn, and Ag; and alkali earth metals including Ca, Sr, Ba, and Mg;the third metal (M) may be one or two or more metals selected among thetransition metals including Ni, Co, Mn, Ti, Cr, Cu, V, Zn, and Ag; orthe third metal (M) may be one or two or more metals selected from thealkali earth metals including Ca, Sr, Ba, and Mg.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,the water-soluble or water-insoluble precursor of lithium may beinjected into the polar solvent so that a molar concentration of lithiumin the polar solvent is 1 to 25 mol/L after the chelating agent isinjected into and dissolved in the polar solvent. In this case, asdescribed above, the water-insoluble precursor of iron and the precursorof phosphate, and selectively, the precursor of the third metal (M) maybe injected into the polar solvent together with the water-soluble orwater-insoluble precursor of lithium so as to satisfy Chemical Formula1.

In the preparation method for a metal phosphate according to theexemplary embodiment of the present invention, stirring may be performedat room temperature for 1 to 30 hours, specifically, 12 to 30 hours, andmore specifically, 16 to 30 hours.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,concentration and drying may be performed by vacuum concentration.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,a colloidal suspension may be prepared by concentrating the polarsolvent containing the chelating agent, the precursor of phosphate, theprecursor of the first metal, and the water-insoluble precursor of thesecond metal by vacuum concentration, and in a case in which thestirring is performed for a long period of time (16 hours or more), thecolloidal suspension may be prepared during the stirring.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,concentration and drying may be continuously performed.

Concentration and drying are continuously performed, which means thatconcentration and drying are continuously performed by vacuumconcentration under constant temperature and pressure conditions, thatis, concentration and drying are performed by a single vacuumconcentration step.

In a case in which concentration and drying are continuously performed,vacuum concentration may be performed at a temperature of 30° C. to 60°C. and a pressure of 20 to 100 mbar. In this case, as a substantialexample, vacuum concentration (that is, concentration and drying) may beperformed for 3 to 24 hours.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,concentration and drying may be discontinuously performed.

Concentration and drying are discontinuously performed, which means thatconcentration is performed by vacuum concentration under constanttemperature and pressure conditions and then drying is performed byvacuum concentration under temperature and pressure conditions differentfrom those at the time of concentration, that is, concentration anddrying are performed, respectively, by a two-step vacuum concentrationstep.

In a case in which concentration and drying are discontinuouslyperformed, concentration may be performed at a temperature of 30° C. to60° C. and a pressure of 20 to 100 mbar, and drying may be performed ata temperature of 30° C. to 60° C. and a pressure of 20 to 100 mbar,independently of concentration conditions. In this case, as asubstantial example, concentration may be performed for 3 to 24 hours,and drying may be performed for 3 to 24 hours.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,the precursor powder obtained by drying may contain a complex of thesecond metal having a valence of 3. In detail, when the water-insolubleprecursor of the second metal is a divalent precursor of iron, theprecursor powder may contain a trivalent complex of iron.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,there is no FT-IR peak by the water-insoluble precursor of the secondmetal on a FT-IR absorption spectrum of the precursor powder, but thereis a peak of the complex in which at least the chelating agent and ironare bonded to each other, and iron in the complex may have a valence(oxidation number) of +3. The complex of iron, having a valence of +3may be formed during the stirring or vacuum concentration step.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,a crushing step of crushing the precursor powder obtained by drying maybe further performed, wherein the crushing step may be performed using ageneral dry or wet crushing method.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,heat-treatment of the powder obtained by drying may be performed under areduction atmosphere or an inert atmosphere. The reduction atmosphereincludes a pure hydrogen atmosphere; or a mixed gas atmosphere in which3 to 10 vol % of hydrogen, and inert gas are mixed with each other. Inthis case, hydrogen or the mixed gas may be supplied at 100 to 500cc/min.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,heat-treatment may be performed at 400° C. to 800° C.

In the preparation method for a lithium composite phosphate-basedcompound according to the exemplary embodiment of the present invention,a crushing step of crushing the powder obtained by heat-treatment may befurther performed, wherein the crushing step may be performed using ageneral dry crushing method.

Preparation Example 1

After preparing an aqueous solution in which 10 wt % of citric acid wasdissolved, lithium phosphate (Li₃PO₄, 99%), iron oxalate (FeC₂O₄.2H₂O,99%), and ammonium phosphate ((NH₄)₂HPO₄, 98%) were weighed and injectedinto 250 mL of the aqueous solution so that a molar ratio of Li:Fe:PO₄was 1.06:1:1.01 and a molar concentration of lithium ion in the aqueoussolution was 23.8 M, followed by stirring at room temperature for 21hours, thereby preparing a precursor solution.

After injecting the prepared precursor solution in a vacuumconcentrator, the precursor solution was vacuum concentrated and driedat 40° C. and a pressure of 28 mbar for 12 hours, thereby obtainingprecursor powder.

The obtained precursor powder was heat-treated at 650° C. for 10 hourswhile supplying hydrogen at 150 cc/min, thereby preparing lithium-ironphosphate.

After crushing the prepared lithium-iron phosphate, 90 wt % oflithium-iron phosphate, 5 wt % of a conducting agent (Super-p+ vaporgrowth carbon fiber (VGCF), 1:1 (weight ratio)), and 5 wt % of a binder(polyvinylidene fluoride (PVdF)) were mixed with each other, and activematerial slurry was prepared using N-methyl-pyrrolidone (NMP) as asolvent.

The prepared active material slurry was applied and dried onto aluminumfoil having a thickness of 17 μm, compacted using a press, and dried at120° C. for 16 hours under vacuum, thereby manufacturing an electrode asa circular plate having a diameter of 12 mm. In this case, an electrodedensity of the electrode was 1 to 2 g/cc.

As a counter electrode, lithium metal foil punched to have a diameter of16 mm was used, and as a separator, a polypropylene (PP) film was used.As the electrolyte, 1M of LIPF_(ε) in a mixed solution of ethylenecarbonate (EC)/ethyl methyl carbonate (EMC) (1:2 (v/v)) was used. Afterthe separator was impregnated with the electrolyte, the separator wasinserted between a work electrode and the counter electrode, andevaluation was performed using a case made of a steel use stainless(SUS) material as a test cell for evaluating the electrode.

Preparation Example 2

Lithium-iron-magnesium phosphate and a test cell were prepared by thesame methods as in Example 1 except for injecting Mg(CH₃COO)₂.4H₂Otogether with lithium phosphate (Li₃PO₄, 99%), iron oxalate(FeC₂O₄.2H₂O, 99%), and ammonium phosphate ((NH₄)₂HPO₄, 98%) into anaqueous solution in which citric acid was dissolved so that a molarratio of Li:Fe:Mg:PO₄ was 1.06:0.98:0.02:1.01.

Preparation Example 3

Lithium-iron-silver phosphate and a test cell were prepared by the samemethods as in Example 1 except for injecting AgCH₃COO together withlithium phosphate (Li:PO₄, 99%), iron oxalate (FeC₂O₄.H₂O, 99%), andammonium phosphate ((NH₄)₂HPO₄, 98%) into an aqueous solution in whichcitric acid was dissolved so that a molar ratio of Li:Fe:Ag:PO₄ was1.06:0.98:0.02:1.01.

Preparation Example 4

Lithium-iron-nickel phosphate and a test cell were prepared by the samemethods as in Example 1 except for injecting Ni(CH₃COOH)₂.4H₂O togetherwith lithium phosphate (Li₃PO₄, 99%), iron oxalate (FeC₂O₄.2H₂O, 99%),and ammonium phosphate ((NH₄) 2HPO₄, 98%) into an aqueous solution inwhich citric acid was dissolved so that a molar ratio of Li:Fe:Ni:PO₄was 1.06:0.98:0.02:1.01.

The following Table 1 illustrates electrical conductivities, BETspecific surface areas, pore volumes, and carbon contents of lithiumcomposite phosphate-based compounds prepared in Preparation Examples 1to 4, and as Comparative Example, characteristics of LiFePO₄ having anaverage particle size of 100 to 300 nm, which was a commercial product,were measured and illustrated in Table 1. In Comparative Example, a testcell was manufactured by the same method as in Preparation Example 1except for changing the cathode active material.

The electrical conductivity was measured using Loresta Equipment(Mitsubishi), the BET specific surface area was measured using Tristar3000 (Micromeritics Instruments Corp.), the pore volume was measuredusing Tristar 3000 (Micromeritics Instruments Corp.), and the carboncontent was measured using an EA 1108 CHNS—O analyzer (FisonsInstruments).

TABLE 1 Electrical Conductivity BET Pore Volume Carbon sample [Scm⁻¹][m²g⁻¹] [cm³g⁻¹] (wt %) Commercial 2.354*10⁻² 13.8789 0.031763 2.44Product Preparation 2.812*10⁻⁵ 36.3615 0.152995 4.20 Example 1Preparation 1.505*10⁻⁴ 42.3995 0.180198 3.10 Example 2 Preparation7.492*10⁻⁵ 33.0205 0.140512 2.73 Example 3 Preparation 1.216*10⁻⁵38.6603 0.157107 2.90 Example 4

FIG. 1 is a view illustrating X-ray diffraction (XRD) characteristics oflithium composite phosphate-based compounds prepared according toPreparation Example 2 and the commercial product corresponding toComparative Example. As illustrated in FIG. 1, it may be appreciatedthat lithium iron magnesium phosphate having an olivine structure wasprepared.

FIGS. 2A to 2D are high-magnification scanning electron microscope (SEM)photographs of lithium composite phosphate-based compounds obtainedafter heat-treatment under a hydrogen atmosphere in Preparation Examples1 to 4, wherein FIG. 2A illustrates a result of Preparation Example 1,FIG. 2B illustrates a result of Preparation Example 2, FIG. 2Cillustrates a result of Preparation Example 3, and FIG. 2D illustrates aresult of Preparation Example 4. As results of SEM observation and XRDpattern analysis of transmission electron microscope, it may beappreciated that lithium composite phosphate-based compounds having openpores formed in primary particles themselves were obtained.

FIGS. 3A to 3D are low-magnification scanning electron microscope (SEM)photographs of the lithium composite phosphate-based compounds obtainedafter heat-treatment under a hydrogen atmosphere in Preparation Examples1 to 4, wherein FIG. 3A illustrates a result of Preparation Example 1,FIG. 3B illustrates a result of Preparation Example 2, FIG. 3Cillustrates a result of Preparation Example 3, and FIG. 3D illustrates aresult of Preparation Example 4. As a result of SEM observation, it maybe appreciated that porous lithium composite phosphate-based compoundparticles having a particle diameter of 1 μm to 200 μm, which weresignificantly coarse, were prepared.

As a result of measuring an average particle size, pore distribution ofthe lithium composite phosphate-based compounds obtained afterheat-treatment under a hydrogen atmosphere in Preparation Examples 1 to4 was illustrated in FIG. 4. As illustrated in FIG. 4, it may beappreciated that the lithium composite phosphate-based compounds hadpores having a bimodal size distribution, including a nano pore havingan average pore size of 3 to 4 nm and a sub-micro pore having an averagepore size of 10 to 25 nm.

FIG. 5A is a view illustrating Fourier transform-infrared (FT-IR)spectra of each of the raw materials used in Preparation Example 2 andFIG. 5B is a view illustrating Fourier transform-infrared (FT-IR)spectra of the dried precursor and the prepared lithium compositephosphate-based compound (Mg doped-LiFePO₄ in FIG. 5). As illustrated inFIGS. 5A and 5B, it may be appreciated that a peak by a ferrous oxalatecomplex(iron oxalate) disappeared and a peak by a ferric ammoniumcitrate complex was formed in the dried precursor.

FIG. 6 is a view showing charge and discharge characteristics(4.3V-2.5V, 0.1 C) of the test cell manufactured in Preparation Example2 and the test cell manufactured using the commercial product. It may beappreciated that in the commercial product, the test cell had dischargecapacity of 154 (mAhg⁻¹), but the cell containing the lithium compositephosphate-based compound according to the present invention as theactive material had discharge capacity of 165 (mAhg⁻¹).

FIG. 7 is a view showing charge and discharge cycle characteristics(4.3V-2.5V, 0.1 C, 28 cycles repeated) of the test cell manufactured inPreparation Example 2 and the test cell manufactured using thecommercial product. It may be appreciated that in the commercialproduct, a change in battery capacity (mAhg⁻¹) at the 25th cycle, basedon battery capacity (mAhg⁻¹) at the 1st cycle (battery capacity at the25th cycle/battery capacity at the 1st cycle×100%) was 98.09%, and inthe test cell according to the present invention, the change in batterycapacity was 99.95%.

Hereinabove, although the present invention is described by specificmatters, exemplary embodiments, and drawings, they are provided only forassisting in the entire understanding of the present invention.Therefore, the present invention is not limited to the exemplaryembodiments. Various modifications and changes may be made by thoseskilled in the art to which the present invention pertains from thisdescription.

Therefore, the spirit of the present invention should not be limited tothe above-described embodiments, and the following claims as well as allmodified equally or equivalently to the claims are intended to fallwithin the scope and spirit of the invention.

1. A porous lithium composite phosphate-based compound comprisinglithium and having open pores formed in primary particles.
 2. Thelithium composite phosphate-based compound of claim 1, wherein aBrunauer-Emmett-Teller (BET) specific surface area of the primaryparticle is 25 to 50 m²/g.
 3. The lithium composite phosphate-basedcompound of claim 2, wherein a pore volume of the primary particle is0.1 to 0.25 cm³/g.
 4. The lithium composite phosphate-based compound ofclaim 1, wherein the primary particle contains nano pores having anaverage pore size of 1 to 5 nm.
 5. The lithium composite phosphate-basedcompound of claim 1, wherein the primary particle contains at leastpores having a bimodal size distribution.
 6. The lithium compositephosphate-based compound of claim 1, wherein it has a compositionrepresented by the following Chemical Formula 1.Li_(1+a)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1) (In ChemicalFormula 1, M is one or more selected from the group consisting of Mg,Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al, and Ag, and x, a,and b are real numbers satisfying 0.00≦x≦1, 0.00≦a≦0.1, and 0.00≦b≦0.10,respectively.)
 7. The lithium composite phosphate-based compound ofclaim 1, wherein a size of the primary particle is 10 nm to 200 m. 8.The lithium composite phosphate-based compound of claim 1, wherein it isa cathode active material for a secondary battery.
 9. A cathode activematerial for a secondary battery, the cathode active material comprisingthe lithium composite phosphate-based compound of claim
 1. 10. Thecathode active material of claim 9, wherein a carbon coating layer isformed on a surface of the primary particle of the lithium compositephosphate-based compound.
 11. The cathode active material of claim 9,further comprising 2 to 6 parts by weight of carbon based on 100 partsby weight of the lithium composite phosphate-based compound.
 12. Acathode for a secondary battery, comprising the cathode active materialof claim
 9. 13. A lithium secondary battery comprising the cathode ofclaim
 12. 14. A preparation method for a lithium compositephosphate-based compound, the preparation method comprising: a)injecting and stirring at least a chelating agent, a precursor ofphosphate, a precursor of a first metal including lithium, and awater-insoluble precursor of a second metal in a polar solvent toprepare a precursor dispersion solution; and b) heat-treating precursorpowder obtained by concentrating and drying the precursor dispersionsolution.
 15. The preparation method of claim 14, wherein the secondmetal is one or two or more metals selected among transition metals, andin the water-insoluble precursor of the second metal, the second metalhas a valence of
 2. 16. The preparation method of claim 15, wherein theprecursor powder contains a complex of the second metal having a valenceof
 3. 17. The preparation method of claim 14, wherein the precursor ofthe first metal is a water-soluble precursor or a water-insolubleprecursor.
 18. The preparation method of claim 14, wherein the secondmetal includes iron.
 19. The preparation method of claim 14, wherein awater-soluble precursor of a third metal, one or more metals selectedamong Mg, Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al, and Agis further injected into the polar solvent.
 20. The preparation methodof claim 15, wherein the lithium composite phosphate-based compoundsatisfies a composition represented by the following Chemical Formula 1.Li_(1+a)Fe_(1-x)M_(x)P_(1+b)O₄  (Chemical Formula 1) (In ChemicalFormula 1, M is one or more selected from the group consisting of Mg,Ni, Co, Mn, Ti, Cr, Cu, V, Ce, Sn, Ba, Ca, Sr, Zn, Al, and Ag, and x, a,and b are real numbers satisfying 0.00≦x≦1, 0.00≦a≦0.1, and 0.00≦b≦0.10,respectively.)
 21. The preparation method of claim 14, wherein theconcentration and drying are performed by vacuum concentration.
 22. Thepreparation method of claim 21, wherein the vacuum concentration isperformed at a temperature of 30 to 60 (Chemical Formula 1) and apressure of 20 to 100 mbar.
 23. The preparation method of claim 14,wherein the heat-treatment is performed under a reduction atmosphere oran inert atmosphere.
 24. The preparation method of claim 23, wherein theheat-treatment is performed at 400 to 800° C.